I debated whether to give this book a 3 or a 4, but in the end I liked it well enough so I went with the 4. Had I the option, I guess it would have gotten a 3.5.
Part of the problem was that the book description doesn't really describe the book, so my expectations weren't met. The only similarity to The Devil in the White City: Murder, Magic, and Madness at the Fair that Changed America, a book I loved, was the World's Fair. The Paris Engima lacked the rich detail and history. The description promised a "series" of grisly murders, of which there were only two, and only one "grisly". Etc.
I liked the writing, and the main character, and when the story was being told the book was quite good and compelling. The problem was that there were numerous case vignettes put forth by the "Twelve Detectives" which had no bearing on the story and actually detracted from it. If the book had just told the one story, it would have been a really good novella, but a lot of what was there felt like filler, or the author wanting to put all his murder mystery ideas into one place.
Glad I read it, but I wouldn't rush out to read more of the same.